registration form with extra calculation (1 post)

I have a site where people will register for an event. I want the to be able to pay in paypal, but BEFORE they need to input different data such as if they have a local tax number choice 1 input the tax, indication if it is the wrong format.

so basically it is a registration form with a calculator for Paypal. I only want them to go to paypal having to pay the price for one item.

I use contact forms 7 which i read is not the best. If I pay I might get 123 or Gravity but for the time being the event and company have not got enough "juice" for me to be able to do that.